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Endemic freshwater sponges (demosponges, Lubomirskiidae) dominate in Lake Baikal, Central Siberia, Russia. These sponges are multicellular filter-feeding animals that represent a complex consortium of many species of eukaryotes and prokaryotes. In recent years, mass disease and death of Lubomirskia baicalensis has been a significant problem in Lake Baikal. The etiology and ecology of these events remain unknown. Bacteria from the families Flavobacteriaceae and Oxalobacteraceae dominate the microbiomes of diseased sponges. Both species are opportunistic pathogens common in freshwater ecosystems. The aim of our study was to analyze the genomes of strains Janthinobacterium sp. SLB01 and Flavobacterium sp. SLB02, isolated from diseased sponges to identify the reasons for their joint dominance. Janthinobacterium sp. SLB01 attacks other cells using a type VI secretion system and suppresses gram-positive bacteria with violacein, and regulates its own activity via quorum sensing. It produces floc and strong biofilm by exopolysaccharide biosynthesis and PEP-CTERM/XrtA protein expression. Flavobacterium sp. SLB02 utilizes the fragments of cell walls produced by polysaccharides. These two strains have a marked difference in carbohydrate acquisition. We described a possible means of joint occupation of the ecological niche in the freshwater sponge microbial community. This study expands the understanding of the symbiotic relationship of microorganisms with freshwater Baikal sponges.

There are significant changes in the consortium of microorganisms of freshwater Baikal sponges during their mass death, which began in 2011. The alleged cause of disease is a significant increase in the number of opportunistic microorganisms. Here, we report the draft genome sequence of Flavobacterium sp. strain SLB02.

A mass bleaching event affecting an endemic Baikal sponge points out the need to study stress responses in these organisms. To get the first data about Hsp70 content changes in endemic Baikal sponge Lubomirskia baicalensis during exposure to elevated temperatures which can occur in its natural habitat, we investigated the effect of two elevated temperatures on Hsp70 levels in the L. baicalensis and its cell aggregates (primmorphs). After increasing the water temperature from ambient 4 to 9 °C Hsp70 levels increased to protect against temperature stress. Incubation at 13 °C for one month resulted in bleaching and death of the sponges. The decreased level of HSP70 was recorded both in the case of bleaching after exposure to 13 °C, and in diseased individuals from the Lake Baikal. The L. baicalensis primmorph temperature stress response was not consistent with that of the sponge. In primmorphs, the Hsp70 level initially decreased at 4 °C and at 13 °C it increased compared with the levels observed in the sponge. Thus, our results show that primmorphs are not a good model system for the study of expression of stress proteins in sponges. Since the temperatures 9–13 °C were observed at L. baicalensis habitats, climate warming may increase vulnerability of this sponge to bleaching.

Genetic and morphological analysis of endemic sponges of the Lubomirskiidae family from Lake Baikal and the upper reaches of the Angara River was performed. Various sponge species acquired a number of similar morphological traits after the transition from the lake into the river. These traits enabled an increase of sponge skeleton strength under the conditions of elevated hydrodynamic activity. The changes significantly impeded morphology-based species identification of Angara sponges. Phylogenetic analysis of ITS regions and noncoding mitochondrial DNA fragments confirmed that the Angara sponges belonged to the Baikalian Lubomirskiidae family and demonstrated the polyphyletic origin of the sponges. The use of combined molecular and morphological data allowed for the clustering of some sponge samples into groups that corresponded to individual species. The absence of genetic isolation between the Baikalospongia intermedia and Lubomirskia baicalensis species was demonstrated, whereas the B. intermedia profundalis subspecies was well separated from B. intermedia. This finding pointed to the necessity of further studies for the clarification of the taxonomic status of this subspecies.

In recent years, the surfaces of endemic Baikalian sponges have exhibited signs of damage and disease such as tissue bleaching, atypical microbial films, and necrotic areas. One of the most common sources of damage on branching sponges Lubomirskia baicalensis is a reddish-brown mucous film fully or partially covering the body of the sponge. Sequencing of the 16S rDNA gene revealed that the film on the surfaces of the sponge consists of Oscillatoriales cyanobacteria belonging to the following genera: Tychonema, Phormidium, and Leptolyngbya, with 44 of the 86 identified sequences belonging to Tychonema. Cyanobacterial fouling of the Baikalian sponge may be caused by changes in environmental conditions of the sponge habitat, in particular, temperature increases and eutrophication of the lakes.

The fatty acid composition of visually health and sick Lubomirskia baicalensis sponges from Lake Baikal (years of 2016 and 2018) was determined. Sponges collected in 2018 had an abnormality of demospongic acid synthesis. Demospongic acids are of antibiotic activity and protect sponges. In vitro, there was an increase in the concentrations of the palmitic, oleic, linoleic and -linolenic sponge acid under stress. These substances are responsible for cell membrane repairing and antibiotically active. During three weeks, there were also demospongic acid synthesis termination and appearance of disease precursors was noted.

Molecular-genetic analysis of cyanophages of the family Myoviridae from the associated community of the endemic Baikal sponge Lubomirskia baicalensis was carried out based on the g20 gene fragment. A high cyanophage diversity according to the g20 marker gene was found in the sponge. The Baikal sponge cyanophages were shown to be similar to those inhabiting plankton. Moreover, specific cyanophage groups that are significantly different from all of the known groups inhabiting the Lake Baikal were revealed.

The first reports on the presence of zoochlorellae within the organelles of mesenchymal cells from freshwater sponges were published in the 19th century. Today, it is well-known that freshwater sponges can be found in association with different endosymbiotic algae. However, until now there has been no detailed information about the endosymbiotic chlorophyll-containing algae in the remarkable endemic green sponges from Lake Baikal. In our study we were able for the first time to isolate and identify endosymbionts from primmorphs cultivated in vitro, and to compare them with those from naturally occurring Lubomirskia baicalensis sponges. Structural as well as molecular biological investigations show that the endosymbiotic alga is a Mychonastes species closely related to M. huancayensis. Another novel aspect of our work was to show that it is possible to use primmorphs of endemic sponges for isolation and subsequent cultivation of their endosymbiotic algae. We employed a simple cold-water (3–4°C) approach fo...

International audience; The morphological description of morphogen-esis accompanying primmorph formation from cell aggre-gates in the endemic Baikal sponge Lubomirskia baicalensis as well as the description of cell composition of these primmorphs was investigated. Development of small sponges from dissociated cells in L. baicalensis is a stepwise process that can be separated into three stages: 1—primary aggregates; 2—early-stage primmorphs; and 3—primmorphs or young sponges. Primary aggregates are irregular in shape and consist of loosely packed cells. Early-stage primmorph development is characterized by cell sorting, aggregate compactization, surface epitheliza-tion and cell differentiation. Primmorphs or young sponges have developed elements of the aquiferous system. The main morphogenesis occurring during primmorph devel-opment in L. baicalensis is the epithelization of the cell aggregate surface accompanied by the flattening of poorly connected spherical cells. The main sources of new exopinacoderm development during L. baicalensis prim-morph formation are choanocytes and amoebocytes that transdifferentiate into exopinacocytes. The cellular com-position of L. baicalensis primmorphs does not differ sig-nificantly when compared to intact sponges. The most abundant cells in both primmorphs and in intact sponges are amoebocytes and choanocytes. Our results provide a morphological basis for future investigations using prim-morphs of this species as models in biotechnology, evo-devo, cytology and molecular biology approaches.

The skeletal elements (spicules) of the demosponge Lubomirskia baicalensis were analyzed; they are composed of amorphous, non-crystalline silica, and contain in a central axial canal the axial filament which consists of the enzyme silicatein. The axial filament, that orients the spicule in its longitudinal axis exists also in the center of the spines which decorate the spicule. During growth of the sponge, new serially arranged modules which are formed from longitudinally arranged spicule bundles are added at the tip of the branches. X-ray analysis revealed that these serial modules are separated from each other by septate zones (annuli). We describe that the longitudinal bundles of spicules of a new module originate from the apex of the earlier module from where they protrude. A cross section through the oscular/apical-basal axis shows that the bundle rays are organized in a concentric and radiate pattern. High resolution magnetic resonance microimaging studies showed that the silica spheres of the spicules in the cone region contain high amounts of 'mobile' water. We conclude that the radiate accretive growth pattern of sponges is initiated in the apical region (cones) by newly growing spicules which are characterized by high amounts of 'mobile' water; subsequently spicule bundles are formed laterally around the cones.

Several unusual overlong-chain unsaturated aldehydes (22:1, 22:2, 23:1, 24:1, 24:2, and 25:2) were found in total lipids of the endemic sponge Lubomirskia baicalensis from Baikal Lake. Tetracos-17-enal was identified as the major aldehyde of the mixture using GC-MS and 1H and 13C NMR spectroscopy. A procedure for the isolation of total overlong-chain aldehydes was suggested. We think that the overlong-chain aldehydes defend the sponge from fouling and predators. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2005, vol. 31, no. 6; see also http://www.maik.ru.

In a preceding study it has been reported that the freshwater sponge Lubomirskia baicalensis, living in Lake Baikal (East Siberia), is composed of spicules forming a characteristic pattern which follows radiate accretive growth. Here we report that the spicules are synthesized by the enzyme silicatein, a protein which is related to cathepsin L. The cDNAs for silicatein and the related cathepsin L were isolated and used as probes to show that the mRNA levels of silicatein in the bases of the spicule skeleton of the animals are low, while the mRNA level of cathepsin L in this region exceeds that of the growing zone. This is the first comprehensive study on the importance of the axial filament/silicatein as an essential structural and functional component determining the growth and stability of demosponge spicules.

In ancient Lake Baikal (East Siberia), freshwater sponges have diversified to an extraordinary degree. The skeleton of Lubomirskia baicalensis, which attains a size of up to 1 m, is constructed from spicules, which are cemented into longitudinal bundles. Our X-ray analysis revealed that the architecture of the specimens follows a highly ordered radiate accretive growth pattern. The spicules have a central axial canal with an axial filament inside. This organic filament is composed of silicatein, the major enzyme involved in silica formation of the spicules. We found that the specific activity of silicatein in samples from the non-growing (basal) zone is much lower than in those from the growth zone (tips) and that even the composition of this molecule differs in these regions. The present study shows for the first time that the turnover of silicatein, the major element of the axial canal of sponge spicules, changes within a sponge specimen depending on the region in which it is found.

1. 1. The free sterol fraction of the Baikalian freshwater sponge Lubomirskia baicalensis was isolated, separated on AgNO 3 impregnated silica gel, and studied by GLC-MS and NMR analyses. 2. 2. It was shown that the sterol fraction contains at least 16 components belonging to the Δ 5 - series ; 14 of them were identified. 25(26)-Unsaturated sterols, 24-norcholesta-5,22-dien-3β-ol and rare sterol baicalosterol (24-ethyl-26-norcholesta-5,22E,25-trien-3β-ol) were found as minor components of L. baicalensis .